Observation of the electromagnetic Dalitz transition $J\mathrm{/}\psi \to e^{+}{e}^{-}{\eta }_c$

Using $(10.087\pm 0.044)\times {10}^9$ $J\mathrm{/}\psi$ events collected with the BESIII detector at the $e^{+}{e}^{-}$ BEPCII collider, we present the first observation of the electromagnetic Dalitz decay $J\mathrm{/}\psi \to e^{+}{e}^{-}{\eta }_c$. The relative branching fraction $R\equiv \frac{\mathrm{\Gamma }(J\mathrm{/}\psi \to e^{+}{e}^{-}{\eta }_c)}{\mathrm{\Gamma }(J\mathrm{/}\psi \to \gamma {\eta }_c)}$ is determined to be $(0.65\pm {0.02}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.07}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{.}})\mathrm{\%}$, where the first uncertainty is statistical and the second systematic. The $q^2$-dependent form factors are also extracted, and no significant deviation from the theoretical prediction is seen.